Histopathological studies will be carried out on a group of eight rhesus monkeys that have been used in a study to determine effects of cannabis on the brain. Of the 10 original monkeys used in the study (two have died of respiratory complications), two monkeys with implanted deep and surface electrodes and two without electrodes were exposed to moderate smoke, two with electrodes were exposed to heavy smoke of inactive marihuana, and two with electrodes were given delta 9-THC intravenously. Of the monkeys prepared with electrodes, only those smoked with active marihuana (heavy and moderate) and the two which received intravenous delta 9-THC consistently developed recording changes during and immediately after the exposure period. After two to three months of repeated exposure, these recording changes have persisted, perhaps representing permanent cellular damage. Each monkey will be exposed to cannabis for six months and then followed for an undefined period of time, depending on persisting recording and behavioral changes. Each animal will then be perfused and its brain studied by light and by electron microscopy to determine if cellular changes underlie the physiological aberrations.